The present invention relates to a polymer-reinforced paper in which the reinforcing polymer is a latex.
It generally is understood that the properties of paper depend largely on the structure of the various fibers that compose the sheet. The two most important structural characteristics are fiber length and cell wall thickness. A minimum length is required for interfiber bonding and length is virtually proportional to tear strength.
Because softwood fibers typically are from about two to about five times longer than hardwood fibers, the former are universally more desired for papermaking. A papermaking stock or furnish also may contain hardwood fibers, but they are present primarily to improve sheet smoothness and formation, e.g., a uniform distribution of fibers in the paper. In fact, the presence of more than minor amounts of hardwood fibers often has a deleterious effect on the strength and tear resistance of the resulting paper. The more common hardwoods employed as a source of fibers include aspen, birch, beech, oak, maple, and gum.
Although not commonly used, eucalyptus (a hardwood) fibers have been employed in paper and paper-related products. For example, a paper reportedly was made from pulp containing from 0.5 to 20 weight percent fine fibrous cellulosic material having an average fiber length of 0.01 mm to 0.4 mm and at least 20 weight percent of pulp made from eucalyptus wood. Other eucalyptus fiber-containing papers also have been reported. Papers made from bleached eucalyptus kraft pulp have been impregnated with a phenolic resin and employed in the manufacture of printed circuit boards. Eucalyptus fibers also have been employed in the manufacture of tissue, including a layered tissue and a tissue impregnated with an oily material. An electrolysis paper containing at least 20 weight percent eucalyptus pulp located between an anode foil and a cathode foil in an electrolytic capacitor has been described. A hard fiberboard material comprising eucalyptus wood has been employed in the manufacture of a high-pressure laminate. A paper web made of poplar or eucalyptus wood and pine wood in a ratio of from 15:85 to 85:15 was coated with a surface layer consisting of a substantially hygroscopic additive. Finally, paper strips based on eucalyptus and pinewood sulphate-cellulose in the ratio of from 50:50 to 10:90 were impregnated with mixtures of aqueous anionic copolymer solutions and dispersions, followed by further surface treatments.
A long-established practice for improving the strength characteristics and durability of a paper involves reinforcement of the paper by polymer impregnation. The polymer employed typically is a synthetic material, and the paper consists primarily of long softwood cellulosic fibers or of a mixture of softwood cellulosic and noncellulosic fibers. Polymer reinforcement is employed to improve one or more of such properties as dimensional stability, resistance to chemical and environmental degradation, resistance to tearing, embossability, resiliency, conformability, moisture and vapor transmission, and abrasion resistance, among others.
In general, the property or properties which are desired to be improved through the use of a polymer-reinforced paper depend on the application. For example, the resistance of a paper to tearing is particularly important when the paper is to be used as a base for masking papers and tapes, abrasive papers for machine sanding, and flexible, tear-resistant marking labels, by way of illustration only. Although strength is a primary attribute, smoothness and good formation also are desired. While significant advances have been made in the improvement of smoothness and formation, opportunities still remain for further improvements in smoothness and sheet formation without sacrificing, or even with improvements in, the strength of papers.